The present invention relates to a power semiconductor device which is used in a power converter or the like.
An essential part cross-sectional view of an active region 20 in which a main current flows in the center of a conventional power semiconductor device and an edge termination structure section 30 that is located at the circumference that encloses the active region 20 is shown in FIG. 14. The edge termination structure section 30 is a part that is provided to reliably hold this OFF-state voltage between the upper and lower electrodes (the lower electrode is not illustrated) of an n-type semiconductor substrate 100 of the device in an OFF state. As the OFF-state voltage increases, there is a need to increase width of the edge termination structure section 30 (the width of the edge termination structure section 30 in a direction extending from the boundary with the active region 20 toward the outer edge of the semiconductor substrate 100). However, because the width of the edge termination structure section 30 is large and the occupied surface area thereof is also large, the surface area of the active region 20 in which the main current of the device flows undergoes a relative decrease. The edge termination structure section 30 is therefore desirably as small as possible.
In the case of the edge termination structure section 30 of a conventional planar semiconductor device, not only is a local concentration of the internal electric field prevented in order to secure the rated forward blocking voltage capability, but a boundary relaxation structure such as a guard ring 40 and field plate 80 or the like is sometimes also provided with the objective of improving the forward blocking voltage capability. Of these boundary relaxation structures, the field plate 80 often employs a metal film of the same type which is formed at the same time as the metal electrode 90, such as an emitter electrode, cathode electrode or the like, which is in contact with a p-type well 70 that is formed on the surface side of the active region 20 of the semiconductor device for the sake of efficiency in the fabrication process. Such a metal film includes an Al thin film or the like to which a minute amount of Si, for example, has been added. Because the thickness of the metal film is from 3 μm to 5 μm in a power semiconductor device, the required emitter and field plate pattern are processed by means of wet etching, which uses a photo process, rather than dry etching. Other field plate materials which can be used are conductive silicon film, and semi-insulating thin film, and so forth.
In addition to improving the forward blocking voltage capability resulting from the electric field relaxation effect, the field plate 80 which employs a metal film of this kind also possesses a function for maintaining long-term reliability of the forward blocking voltage capability function by shielding unnecessary charge (referred to as ‘external charge’ hereinbelow) which is harmful and applied from the external environment to the insulating film on the surface of the edge termination structure and the atmosphere external to the device.
A semiconductor device in which edge termination structures that alternately comprise an electrically conductive field plate and a semi-insulating field plate is well known. Such a semiconductor device has a stable high forward blocking voltage capability, even when the surface area occupied by the edge termination structure is small. Japanese Patent Registration No. 3591301 is a well-known document relating to the field plate technology for a power semiconductor device.
However, as mentioned earlier, because the required pattern is processed by wet-etching a metal film that has a relatively large film thickness, with the field plate that is conventionally applied to a power semiconductor element, the amount of side etching of the metal film is large. Because it is necessary to anticipate each of the widths indicated by the arrows in FIG. 14 in the design by taking the width corresponding with this large amount of side etching as the design margin, it is difficult to shorten the edge termination structure. In addition, scatter readily occurs in the initial characteristic of the device forward blocking voltage capability because, in comparison with dry etching, wet etching has a low etching accuracy due to the large scatter in the etching amounts and so forth.